The present invention relates to a method for producing iron carbide from an iron-containing feed material. More specifically, the present invention utilizes a two step process to convert iron oxide to metallic iron in the first step and metallic iron to iron carbide in the second step for use in steel-making.
The steel industry has relied on a process that has been in use for many years for the conversion of iron ore into steel. The process converts iron ore into pig iron in a blast furnace using coke produced in a coke oven. The process next converts the pig iron or hot metal into steel in an open hearth or basic oxygen furnace.
In recent years, federal and local environmental regulations have caused numerous problems for steel producers using this steel-making process. The blast furnace and coke ovens used in the process are not only energy intensive but also responsible for most environmentally damaging emissions by steel producers. To redesign or modify blast furnaces and coke ovens to comply with pollution standards is expensive. The expense would cause the cost of steel produced by the conventional steel-making process to be non-competitive with steel produced by foreign competitors.
To address these problems, a process was developed for steel production that eliminates the blast furnace and coke oven in the steel-making process. In the process, a bed of iron oxide is fluidized by a single, multiple-component gas stream and directly converted into an iron carbide-containing product, primarily consisting of Fe3C. The iron carbide is then added to a basic oxygen or electric arc furnace to produce steel. In the process, reduction and carburization reactions occur together in the same fluidized bed.
Another process has been applied to produce acicular iron carbides having desired magnetic characteristics for use in magnetic recording and as catalysts for converting CO and H2 into lower aliphatic hydrocarbons. In the process, a bed of the acicular iron oxide is reduced by one gas and a bed of the reduced product is then carburized by another gas to produce acicular iron carbides of the form Fe5C2. The process suffers from slow reaction kinetics, large amounts of impurities (including iron oxide, free carbon and metallic iron) in the acicular iron carbide product, and poor gas efficiency (i.e., poor utilization of reactants in gas). The product, Fe5C2, is quite unstable and requires more carbon reagent to form than Fe3C (and is therefore more expensive to produce).
Other techniques to convert an iron-containing feed material into an iron carbide-containing product require expensive components, suffer from poor gas efficiency, and/or raise other operational complications.
It would be advantageous to provide a process to convert iron-containing materials into iron carbide that has a high gas utilization. It would be further advantageous to produce an iron carbide product with environmentally friendly and/or non-hazardous byproducts. It would be a further advantage to optimize the reaction kinetics of chemical reactions to convert iron-containing materials into iron carbide and to produce an iron carbide product that has high purity and low residual iron oxide.
Additionally, it would be advantageous to develop an environmentally friendly, energy efficient and inexpensive process to produce steel. It would be further advantageous to convert, inexpensively and efficiently, iron-containing materials into iron carbide for use in the production of steel.
In accordance with an embodiment of the present invention, a two step process for producing iron carbide from an iron oxide-containing feed material is provided. As used herein, xe2x80x9ciron carbidexe2x80x9d preferably includes Fe2C and Fe3C, and xe2x80x9ciron oxidexe2x80x9d preferably includes FeO, Fe2O3, and Fe3O4. In the first (reduction) step, a feed material containing iron oxide is converted to an intermediate product by contacting the feed material with a reducing gas to reduce the iron oxide to metallic iron, and in a second (carburization) step the metallic iron is converted into an iron carbide product.
The reducing gas preferably contains sufficient hydrogen gas, the primary reducing agent, to perform substantially complete reduction of the iron oxides in the feed material to metallic iron. Typically, the reducing step is a closed circuit process so that virtually all of the reducing reagent is utilized by the process to remove oxygen from the feed material. Preferably, the predominant component of the reducing gas is hydrogen gas, and more preferably the reducing gas contains at least about 80 mole % hydrogen gas. Water, the byproduct of the reduction reaction, is easily removed from the first step off-gas by suitable techniques.
At least most of the iron in the intermediate product is in the form of metallic iron. Preferably, at least about 70 and more preferably at least about 90 mole % of the iron in the intermediate product is in the form of metallic iron. The intermediate product typically contains no more than about 35 mole percent iron carbide, more typically no more than about 25 mole percent, and more typically no more than about 10 mole percent iron carbide.
It is preferred that iron oxide be at least about 90 mole percent of the feed material in the first step on a water free basis. Preferably a substantial portion, and more preferably at least most, of the iron oxide in the feed material is converted to metallic iron in the first (reduction) step. The presence of iron oxides in the intermediate product is not desired since iron oxide can slow the reaction kinetics in the carburizing step and lengthen the necessary residence time of the material in the carburizing step for a desired degree of carburization.
In the carburization step, the intermediate product is contacted with a carburizing gas to produce an iron carbide product. The carburizing gas includes carbon monoxide and hydrogen gas. Preferably, the carburizing gas contains at least about 5 and more preferably at least about 15 mole % carbon monoxide and at least about 80 mole % hydrogen gas.
The carburizing gas can also include other components such as carbon dioxide, methane, water vapor and a diluent such as nitrogen or another inert gas. Preferably, the carburizing gas includes no more than about 5, more preferably no more than about 3, and more preferably no more than about 1 mole % carbon dioxide; preferably no more than about 15, more preferably no more than about 10, and more preferably no more than about 5 mole % methane; preferably no more than about 10, more preferably no more than about 1, and more preferably no more than about 0.5 mole % water vapor; and no more than about 10 mole % inert gases.
As will be appreciated, the temperatures of the carburizing gas and of the bed of the intermediate product during carburization are important to the reaction kinetics. Preferably, the carburizing gas has a gas temperature of at least about 550xc2x0 C. and the intermediate product a bed temperature of at least about 500xc2x0 C.
Because of the high concentration of carbon monoxide in the carburizing gas and the fact that the carbon monoxide directly converts metallic iron into iron carbide, less gas is required for complete carburization than in suitable two-step processes and the rate of the carburization reaction is relatively high. Compared to other two-step processes, the process of the present invention requires lower capacity components for a given throughput of feed material. The reduced capacity components significantly reduce capital and operating costs and water consumption.
At least most of the carbon monoxide in the carburizing gas is passed through the intermediate product only once (i.e., the carburizing step is preferably an open circuit while the reducing step is preferably a closed circuit). As used herein, a closed circuit means that at least most and more typically at least about 90 mole % of the reducing off-gas is recycled to the reducing step, and an open circuit means that no more than about 50 mole % and more typically no more than about 10 mole % of the carburizing off-gas is recycled to the carburizing step. Stated another way, at least most of the unreacted carbon monoxide in the carburizing gas is not recycled to the second step. Preferably, at least about 30%, more preferably at least about 50%, and more preferably at least about 65% of the carbon monoxide in the carburizing gas is reacted with the intermediate product in the single contact of the carburizing gas with the intermediate product. No more than about 30, more preferably no more than about 20, and more preferably no more than about 10 vol % of the carburizing off-gas is recycled to the carburizing step because if too much of the off-gas is recycled methane will build up in the carburizing gas and dilute the carbon monoxide concentration to relatively low levels.
The use of the carbon monoxide in the carburizing gas for only a single pass through the intermediate product is made economical at least in part by the use of the second step off-gas as a fuel source in other steps of the process, e.g., heating of the feed material and/or the reducing and/or carburizing gases prior to contacting the gases with the feed material and intermediate product, respectively. Preferably, at least about 80%, more preferably at least about 90% , and more preferably all of the carbon monoxide in the carburizing off-gas is used as a fuel source in one or more steps of the process. Typically, these preferred percentages of carbon monoxide represent no more than about 50, more typically no more than about 40, and more typically no more than about 30% of the carbon monoxide in the carburizing gas. The lower fuel costs offset the higher expense associated with forming more carburizing gas. Additionally, hydrogen gas can be separated from the off-gas of the carburizing step to reconstitute the reducing gas in the reducing step.
At least most of the iron carbide product is preferably iron carbide. It is desired that at least about 90 mole percent, and more preferably at least about 95 mole percent of the iron carbide be in the form of Fe3C. Fe2C is not desired as it, unlike Fe3C, is highly reactive and will oxidize upon exposure to air. Preferably, the iron carbide product contains no more than about 25 and more preferably no more than about 5 mole percent impurities, including metallic iron, free carbon, and iron oxide. Impurities such as metallic iron, free carbon, and iron oxide can cause problems if the iron carbide product is converted into steel and the steel processed into useful articles. By way of example, metallic iron in the iron carbide product can oxidize to form iron oxides which create difficulties in converting the iron carbide product into steel.
The high level of product purity is made possible by monitoring the composition of the carburizing off gas. Because methane is typically a minor component in the carburizing feed gas, it is practical to monitor the methane concentration in the carburizing reactor off gas to control the product quality. An increase in the methane concentration in the carburizing reactor off gas indicates that either 1) the metallic iron in the reactor has been converted to such a degree that there is no longer sufficient metallic iron present to pursue the preferred reaction (of forming iron carbide to consume the carbon monoxide), allowing the unconsumed carbon monoxide to react with the hydrogen in the process gas to form methane and water vapor which can then be observed in the reactor off gas, or 2) the process conditions (such as the temperature, pressure and/or process feed gas composition) have been altered to allow the iron carbide product in the reactor to back react with the hydrogen in the process gas to form methane and metallic iron. When or at some time before enough metallic iron in the intermediate product has formed iron carbide to permit the carbon monoxide in the process gas to back react and form methane, or before the iron carbide product itself can back react with the hydrogen in the process gas, the iron carbide product is removed from the reactor and more intermediate product is added to the reactor to increase the concentration of metallic iron and allow for the carbon monoxide in the process gas to form additional iron carbide.
In one embodiment, the iron carbide product is fed directly to an appropriate reactor for conversion into steel.
In one embodiment of the present invention, the process is a continuous process. Preferably, the two process steps are conducted in separate reaction zones to facilitate the continuity of the process. Preferably, in one or both process steps the reaction zone is a fluidized bed.
The present invention can have numerous advantages over existing methods besides those advantages discussed above. One embodiment of the present invention advantageously provides a continuous process to convert the iron-containing materials into iron carbide. The present invention thereby avoids the increase in operating expenses associated with batch processes.
Another embodiment of the present invention advantageously provides a process with rapid reaction kinetics. The composition of each gas can be selected to optimize the kinetics of the reaction in each process step. The reaction conditions, such as pressure, temperature, and gas compositions, can also be selected to optimize the kinetics of each reaction.
Another embodiment of the present invention advantageously provides a process that produces an iron carbide product of high purity. The iron carbide product is substantially free of impurities, including free carbon, iron oxide, and metallic iron.
Another embodiment of the present invention advantageously produces byproducts that are environmentally friendly and nonhazardous. The chief byproduct is water vapor.
Another embodiment of the present invention advantageously provides an environmentally friendly, energy efficient, and inexpensive process to make steel. The process eliminates the blast furnace and coke oven by direct conversion of iron-containing materials to iron carbide followed by the production of steel.